To assist in troubleshooting commonly seen email server errors we have created a list of error codes and next steps to fix the issue.
421 Unexpected failure, please try later:
Error on the email server's side. We typically will hold mail to try to relay it to the server, but it can't go through because every attempt receives this error.
Solution: check your mailserver logs for errors indicating the cause
Legitimate and well-configured mailservers will automatically retry shortly, and mail will go through.
Solution: It’s a “450” error - that’s a temporary deferral. The server shouldn’t be returning the email to the sender. It should be retrying.
450. Requested mail action not taken: mailbox unavailable. SMTP server could not access a mailbox to deliver your message:
This could be caused by a process on the remote server tidying up the mailbox, or the remote mailbox could be corrupt, or the remote mailbox may be stored on another server which is currently offline, or the network connection went down while sending, or the remote mail server does not want to accept mail from your server for some reason (IP address, blocklisting, etc..). The next attempt to send by your server may prove successful.
452 4.3.1 Insufficient system resources:
This is usually indicative of insufficient diskspace - if not on the volume that actually holds the mailstore (mail partition), then on a different volume that holds logging data (partition on which logs are kept).
Solution: Clear diskspace. Check out this blog posting - it talks about backpressure settings on MS Exchange servers:
550 Error - SMTP Authentication error:
If you're using Cpanel you need to click the check box for your server to accept the mail for your domain.
Solution: If you're using cPanel, take a look at Step 4 in this article in our KB. https://support.mailroute.net/hc/en-us/articles/224059348-cPanel
You will be looking for the Mail Exchanger section which will give you the options of Auto-Detect, Remote and Local. You need to select Local then click Change. Once that is done, log out of the DNS cPanel and log back in to make sure that Local is still selected.
And be sure to check your cpanel settings on the server as well, of course.
If you're not using cPanel, take a look at your email server configuration. It needs to know that it's the authoritative server for your domain.
550-not permitted to relay through this server.
Solution: Please turn on SMTP Authentication in your mail client, or login to the 550-IMAP/POP3 server before sending your message.
550-5-7-1 in o365:
This error occurs when the recipient's domain has security or policy settings that reject the sender's message. However, we were unable to determine the specific setting that's causing this rejection. Usually the error is reported by an email server outside of Office 365. Common issues include the following: the receiving server suspects the message is malicious or spam; the Sender Policy Framework (SPF) record for the domain is incorrectly configured or doesn't exist; or the message includes an attachment larger than the receiving server will accept. Try one or more of the following:
Solution: Review the reported error - Check the Reported error shown below to help determine what the issue might be. For example, if the issue is related to an SPF failure, the reported error will usually include the acronym "SPF" or the phrase "Sender Policy Framework."
Correctly configure your SPF records - If you're the sender's email admin, make sure your domain's SPF records at your domain registrar are properly configured. Office 365 supports only one SPF record (a TXT record that defines SPF) for your domain. Include the following domain name: spf.protection.outlook.com. If you have a hybrid configuration (some mailboxes in the cloud and some mailboxes on premises) or if you're an Exchange Online Protection standalone customer, add the outbound IP address of your on-premises servers to the TXT record. To learn how, see Customize an SPF record to validate outbound email sent from your domain (https://technet.microsoft.com/en-us/library/dn789058(v=exchg.150).aspx) and External Domain Name System records for Office 365 (https://support.office.com/en-us/article/External-Domain-Name-System-records-for-Office-365-c0531a6f-9e25-4f2d-ad0e-a70bfef09ac0#BKMK_SPFrecords) .
550 5.7.1. Unable to relay:
If the Reported error indicates a problem with relaying (e.g. "unable to relay"), then the email server that reported the error likely isn't set up correctly to receive and relay messages from the sender's domain. This server will usually be one of your on-premises servers in a hybrid environment, a smart host email service that you're trying to route messages through, or possibly even an email hosting service you used in the past yet still have mail flow settings pointing to (e.g. your MX record at your domain registrar still points to your previous email service provider). Check Error reported by shown below to determine what domain, service, or server is reporting the error. The email server needs to be configured to either accept messages from anonymous users or to include the sending domain or IP in its list of authenticated senders. On an Exchange server, you can set this up in the server's receive connector. If it's a smart host managed by another service or partner, contact the service or partner to configure their servers to accept and relay messages from your senders. Also, work with your domain registrar to make sure your MX records are properly configured.
Contact the recipient's email admin - For some scenarios, you can fix the issue by contacting the email admin at the recipient domain to ask them to add the sender's email address or your domain to their allowed senders list, or to relax the setting that's causing the rejection.
For more information and tips for fixing this issue, see Fix email delivery issues for error code 5.7.1 in Office 365 (http://go.microsoft.com/fwlink/?LinkId=389365) .
550 5.1. User Unknown error message:
All users need to be authenticated with Exchange
General Information about SMTP Error Codes:
- 250 – This SMTP server response simply means everything went well and your message was delivered to the recipient server.
- 400XX– Are temporary errors
- 500XX –Are generally bounce errors